kurtselva

I am doing a battery life indicator for a device. A red led will light up when the voltage reaches below a certain limit, which is at least 2 days before the AA battery fully dies.

As such, I want to know how to determine the number of days left before a AA battery dies. I have attached the datasheet for a AA battery but it only shows the constant current performance chart. My device draws around 50ma current for about 10seconds, 4 times a day, when a motor rotates. And all along, it draws a current (ard 1ma I think) when it runs a timer.

So, how can I determine the voltage when the battery is abt 48hours before "dying"?

Have you noticed how wrong your laptop or tablet gets this. They have vastly more engineering resource than you and they can't get it right. It is not something that ca be accurately predicted. There are too many variables.

I think half the trouble is how they estimate the upcoming use. The might have a good handle on Ah left (but I wouldn't know if they do or not) but they seem to take the instantaneous consumption and assume you'll be doing that non-stop until it dies. So if it looks like you have a long time left, as soon as you do something intensive for a few moments, your time left drops drastically based on that heavy load.

Have you noticed how wrong your laptop or tablet gets this. They have vastly more engineering resource than you and they can't get it right. It is not something that ca be accurately predicted. There are too many variables.

Predicting "remaining life" is difficult.

OTOH knowing when it's "nearly dead" isn't. Most batteries go down to about 1.1V then drop off very quickly (think "cliff").

You need to put a small load on the battery for about 5-10 seconds then measure the voltage, eg. A transistor+resistor that lets 50mA pass. Switch the load on, wait, measure the voltage, switch the load off. If it's 1.15 volts or less, start flashing the warning LED. You could do this once per day.

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

I think half the trouble is how they estimate the upcoming use. The might have a good handle on Ah left (but I wouldn't know if they do or not) but they seem to take the instantaneous consumption and assume you'll be doing that non-stop until it dies. So if it looks like you have a long time left, as soon as you do something intensive for a few moments, your time left drops drastically based on that heavy load.

EXACTLY! That is also valid for cars. If my car tells my I have 50km worth of fuel left, I make that fuel last for at least 70km, because I will change the way I drive. Obviously the indicator will change as well, and increase the autonomy indication, which is a lot more than Windows does calculating remaining battery time.

Like it's been said, you can get at most a fair estimate, but not an actual precise prediction.

If you are trying to calculate remaining battery time on a device for which the power consumption doesn't change much over time and doesn't depend much on how humans use it, then you can get a very good estimate. In other words, if the consumption curve for the device is known and doesn't change much, calculating remaing battery time is pretty easy.

What you are attempting to do is to calculate the mAH charge remaining in the battery at any given time from when the battery last had a complete charge. To do this accurately requires that one reads the actual real time current consumption continuously and keeps an accumulated mAH consumed. This is then subtracted from the rated full mAH capacity of the battery. It can be done but it requires current sensing/measuring hardware and extra software.

OTOH knowing when it's "nearly dead" isn't. Most batteries go down to about 1.1V then drop off very quickly (think "cliff").

Indeed - any you need to know the actual chemistry of the battery involved, AA cells might be Zinc/carbon, alkaline,NiMH or even NiCd or NiZn! Unless you know the chemistry you can't know which voltage represents the death-knellof the cell. For NiCd for instance its such a flat curve that in practice you can't tell, fortunately with NiMH you canfairly well. So you probably need to decide on a single type of cell to get a reasonably reliable indication.

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